Low frequency loudspecker enclosure with configurable directivity

ABSTRACT

This invention relates to a low frequency loudspeaker (sub woofer), which comprises at least one pair of loudspeakers mounted in the same box, facing in opposite directions and powered as separate elements by signals coming form a single source but having different amplitude and phase.

FILED OF INVENTION

This invention regards loudspeaker enclosures, in particular for lowfrequencies.

STATE OF THE ART

More than in the past, today's requirements for the reproduction ofmusic and speech, for live applications and professional or otherpurposes require the use of enclosures suited to achieving theobjective.

These enclosures have increasing need for control of their emissiondirectivity, to ensure high quality high-level reproduction only in therequired areas.

It's a well-known fact that directivity control is linked with thedimensions of the sound system, or prior to that, with those of thespecific elements it's made up of. These dimensions are in turn linkedwith the wavelengths of the frequencies or frequency bands that theaforementioned elements must reproduce. For example, controlling afrequency band from 1000 Hz to 20000 Hz in order to measure theattenuation of 6 dB at the required dispersion, the parameterconventionally chosen to identify the dispersion of an enclosure orsound reinforcement system, means that the component, loudspeaker orsystem which must control the angular emission of this band must have adimension of at least 2λ of the minimum frequency to be controlled onthe plane(s) on which this control has to be carried out: in thisexample, no less than 68 cm. (344/1000*2)

Without going into the details of a description of the various solutionssought to achieve this objective, on which abundant technical/commercialdocumentation already exists, as the dimensions necessary for mid andhigh frequencies are still limited, the aim of achieving highdirectivity has been amply achieved using various types of hornenclosures or more recently in vertical line array configurations, withtheir parameters controlled by DSP (Digital Signal Processors), able tomake these modular enclosure configurations assume different angulardispersion patterns.

For the mid low, low or sub low frequencies on the other hand, obtaininghigh directivity is a problem which isn't easily solved, due to thelarger dimension the enclosures must in some way assume when one wantsto control these frequencies, dimensions that become increasingly largerthe lower the bands of frequencies to be reproduced with narrow angulardispersion become.

For example, if its necessary to control a frequency band starting from100 Hz in an enclosure or system of enclosures (in this case not onlyvertical line arrays, but also and more simply, groups of enclosuresclosely coupled together), the dimensions required are very large. Infact, 2λ of 100 Hz is 6.8 m, a dimension that for an enclosure, or evena group of enclosures, is obviously rather unmanageable from all pointsof view. If one then considers that although classified as lowfrequencies, frequencies such as 100 Hz are not the only ones which mustbe reproduced at high sound level in the professional field, and 50/40or 30 Hz must also be reproduced with the same energy in order to ensurecomplete sound reproduction, it's obvious that the dimensions soundsystems or enclosures would need to have to control their directivitybecome even larger; for example, 2λ of 30 Hz is approximately 23 m!

This “normally” also happens in the case of horn enclosures or systems,sometimes preferred where large dimensions are not a hindrance to theiruse, for greater transduction efficiency, not only for direct radiationmodels because, in both cases, the directivity (or rather the graduallack of directivity as frequencies become lower and lower) isestablished by the well known phenomenon of diffraction.

It's therefore obvious that to control dispersion at low frequenciesit's necessary to take another direction or integrate the techniquealready known, essentially regarding the dimensions of a horn or thedimensions of the baffle in a direct radiating system (or even indirectradiating, such as a passband system), with other “expedients” aimed atincreasing directivity.

Well known examples of these “expedients” can be found in numerousacoustic engineering books by authors who made audio history, such asOlson and Beranek, to mention just the most famous. These “expedients”are based on interference methods, which were and still are appliednowadays to microphones, for which it has always been necessary tocontrol their directivity in relation to the signal they pick up forvarious reasons, such as immunity from background noise from directionsother than that of the sound they're used to pick up, capacity to pickup weak far-off sounds, insensibility to feedback, etc.

Through the years, some projects of low frequency enclosures using these“expedients” have been proposed, without however meeting with particularsuccess, perhaps due to a question of all-round performance and/orperformance:price ratio.

Recently however there has been a revival of these techniques, due torequests for limiting dispersion at low frequencies becoming more andmore insistent, but also the possibility of applying to theaforementioned systems the necessary sophisticated “regulations”suitable for controlling directivity by means of “interference”, whichmodern electronics and use of DSP units for audio processing enable tobe easily done at relatively low costs.

In fact, having the possibility of applying different delays or phaseshifts to the sound sources used together to obtain a given directivityis no longer a costly almost unattainable procedure as it was generallyspeaking even just about ten years ago.

Systems have therefore appeared on the market for the reproduction oflow frequencies that apply techniques such as those described and areable to reproduce audio signals starting from 30/40 Hz with dispersionpatterns similar to those obtained in various types of microphones, suchas cardioid, super-cardioid or hyper-cardioid to mention the best known.

These enclosures were built, with the appropriate variations, solutionsadopted for microphones were used, regarding which there's a great dealof detailed literature, as microphones are the inverse of loudspeakers.Regarding this, for information purposes and as a significant example,it's worth mentioning a study by Marinus M. Boone and Okke Ouweltjespublished in the JAES Vol. 45 N° 9, Sep. 1997, entitled “Design of aLoudspeaker System with a Low-Frequency Cardiodlike Radiation Pattern”.

AIMS AND SUMMARY OF INVENTION

One aim of this invention is to propose and supply a new, originalset-up for a dedicated loudspeaker enclosure, able to give polardispersion at low frequencies, no longer with a distribution ofpractically 360°, but according to needs, changed into one or more ofthe various listed dispersion patterns.

Yet another of the invention's aims is to supply a sound system for lowfrequencies that has a simplified construction design, enabling it to beused as a traditional system, but when necessary, controlledelectronically to vary its electro-acoustic configuration and obtain thecontrolled dispersion described above.

Then another aim of the invention is to build an improved speakerenclosure for low frequencies with configurable directivity, at the sametime maintaining very compact overall dimensions, great constructionsimplicity and user-friendly regulation which keep costs low andprice:performance:directivity ratio high.

These aims have been achieved, in accordance with the invention, with aspeaker enclosure for low frequencies at least according to claim 1.

BRIEF DESCRIPTION OF DRAWINGS

The innovation proposed here will be described in greater detail in thecontinuation and referring to the attached designs, which are given asan indication, are not restrictive and in which:

FIG. 1 shows a front view of the low frequency speaker enclosure(subwoofer) with variable directivity;

FIG. 1A shows the cross-section of the enclosure according to the A-Aarrows in FIG. 1;

FIG. 1B shows another cross-section of the enclosure according to theB-B arrows in FIG. 1;

FIG. 2 shows a constructional variation of the enclosure according tothe invention;

FIG. 3 shows an acoustic and electric operating diagram of the systemaccording to the analogy of the equivalent electric circuits;

FIGS. 4, 5 and 6 show the same number of horizontal/vertical polarcurves of the low frequency loudspeaker system in compliance with theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The low frequency enclosure or subwoofer shown is equipped with twoloudspeakers (Spk1 & Spk2) fitted in a single box (V1) with very compactdimensions, for example 70×60×50 cm. The two loudspeakers can also, withno limitations, be different from one another (e.g. one 18″ and theother 15″) and point in opposite directions, one towards the front, theother towards the back. Inside the box (V1), on at least two sides ofthe front loudspeaker two conduits (P1, P2) are foreseen, while the rearloudspeaker faces into a chamber (V2) with side openings (P3, P4, P5).Basically, the two loudspeakers have the same volume of air and the samegenerator (FIG. 3) in common, but are powered as separate elements withsignals which have a different amplitude and phase by two separateamplification circuits (11, 12) which include, among other things, anelectronic delay circuit 13, 14.

If necessary when omnidirectional low frequency dispersion isn'trequired, this geometric layout (when electronically assisted) FIG. 4allows a great attenuation of rear emission, by exploiting theinterference phenomenon from 30/40 Hz up to frequencies of over 100/120Hz, giving dispersion patterns like those shown in FIGS. 5 and 6.

In particular, FIG. 4 shows the horizontal/vertical polar curves of thesubwoofer system measured in half space (2π) with the microphone at fourmeters without activating the system for the attenuation of rearemission at low frequencies according to this invention.

It should be noted that the system is basically omnidirectional over theentire operating band. Amplitude (parallel lines)=1 dB. Degrees(Meridians)=10°.

FIG. 5 shows the system subwoofer horizontal/vertical polar curvemeasured in half space (2π) with the microphone at four meters andactivating the system for the attenuation of rear emission at lowfrequencies according to this invention.

It should be noted that the system has cardioid-type rear radiation overthe entire operating band. Amplitude (parallel lines)=1 dB Degrees(Meridians)=10°.

FIG. 6 shows the horizontal/vertical polar curves of the subwoofersystem. For the 63 Hz ⅓ octave band, the polar responses measured inhalf space (2π) with a microphone at four meters are compared directly:

-   -   Omnidirectional, without activating the system for rear        attenuation of low frequencies,    -   Cardioid-like, activating the system set for this dispersion        pattern,    -   Hypercardioid-like, activating the system set for this        dispersion pattern.

Note must be taken of the great difference in energy that the systemgives at the rear at low frequencies according to the various dispersionpatterns obtained with the dedicated settings (for convenience, 63 Hzwas taken as centre ⅓ octave of the reproduced band). Amplitude(parallel lines)=1 dB. Degrees (Meridians)=10°.

Its compact dimensions and low weight also make it an element that canbe easily stacked or flown in multiples to form vertical line arrays,but can also more traditionally be installed close together in multiplesto form horizontal line arrays or in any case groups of acousticallycoupled elements and operating in “piston band” conditions.

Another important aspect of this in the invention consists in thepossibility of varying the construction geometry without altering itssimplicity, in order to be able to modify the entity of the acousticparameters involved, established by volumes and dimensions of theconduits or apertures, in order to obtain acoustic performance whichdiffers from the point of view of amplitude and passband reproduced.This other aspect is shown in FIG. 2, in which the same alphanumericaldata with the addition of (′) to show parts which are identical orequivalent to those shown in FIG. 1 have been used.

1. A low frequency loudspeaker enclosure (subwoofer), characterized bythe fact that it includes at least one pair of loudspeakers mounted inthe same box, facing the opposite directions and powered as separateelements by signals coming from a single source but having differentamplitude and phase.
 2. The low frequency loudspeaker enclosureaccording to claim 1, in which a first loudspeaker faces towards thefront and a second loudspeaker towards the rear, in order to send thesound in the opposite directions, said loudspeakers being identical ordifferent from one another and driven by amplification circuits, eachincluding an electronic delay circuit.
 3. The low frequency loudspeakerenclosure according to claim 1, in which two open conduits are providedon at least two sides of the front loudspeaker, and in which the rearloudspeaker faces on to a chamber having two side openings.
 4. The lowfrequency loudspeaker enclosure according to claim 3, in which saidfront loudspeaker is contained in the box or protrudes from the front ofit.
 5. The low frequency loudspeaker enclosure according to claim 3, inwhich said conduits and/or side apertures have variable dimensions tomodify the system's acoustic parameters.
 6. The low frequencyloudspeaker enclosure according to claim 1, which can be placedalongside or stacked on other enclosures to form horizontal and verticalarrays, or coupled and installed one above the other in multiples toform groups operating in a “piston band” set-up.
 7. Method ofconstructing a low frequency loudspeaker enclosures which comprises: theuse of at least a pair of loudspeakers, mounted in the same box, facingin opposite directions compared to the sound emission, one facingforward and the other backwards on to a chamber having side openings;the powering of the aforementioned loudspeakers as separate elementswith signals, coming from a single source but with different amplitudeand phase, by using separate amplification circuits, each including anelectronic delay circuit, and the possibility of varying the reciprocalentity of the enclosure's acoustic parameters, modifying the load volumeof the loudspeaker and/or dimensions of the conduits/apertures on thefront and/or apertures positioned at the rear, in order to obtaindifferent dispersion patterns.
 8. The low frequency loudspeakerenclosure according to claim 2, in which two open conduits are providedon at least two sides of the front loudspeaker, and in which the rearloudspeaker faces on to a chamber having two side openings.
 9. The lowfrequency loudspeaker enclosure according to claim 4, in which saidconduits and/or side apertures have variable dimensions to modify thesystem's acoustic parameters.
 10. The low frequency loudspeakerenclosure according to claim 2, which can be placed alongside or stackedon other enclosures to form horizontal and vertical arrays, or coupledand installed one above the other in multiples to form groups operatingin a “piston band” set-up.
 11. The low frequency loudspeaker enclosureaccording to claim 3, which can be placed alongside or stacked on otherenclosures to form horizontal and vertical arrays, or coupled andinstalled one above the other in multiples to form groups operating in a“piston band” set-up.
 12. The low frequency loudspeaker enclosureaccording to claim 4, which can be placed alongside or stacked on otherenclosures to form horizontal and vertical arrays, or coupled andinstalled one above the other in multiples to form groups operating in a“piston band” set-up.
 13. The low frequency loudspeaker enclosureaccording to claim 5, which can be placed alongside or stacked on otherenclosures to form horizontal and vertical arrays, or coupled andinstalled one above the other in multiples to form groups operating in a“piston band” set-up.